This invention relates generally to an electrophotographic printing machine, and more particularly, concerns a paddle wheel sheet feeding system for feeding sheets from a stack along a predetermined path.
Many of the prior art paddle wheel sheet feeding mechanisms occasionally malfunction, feeding more than one sheet at a time or failing to feed a sheet on demand. Consequently, the search has continued for a more consistent and reliable feeder mechanism.
One of the problems with existing feeders that employ paddle wheel separation is that they develop normal force as a result of impacting the copy sheets and by progressive bending of the paddle wheel blades during the sweep through their arc of contact. The paddle wheel blades are usually thin and produce unstable vibrations at certain revolutions per minute causing a large impulse when they first strike the sheets and generate an increasing normal force during their arc of contact.
The large impulse generated by the blades of the paddle wheel striking the sheets at the beginning of their arc of contact can actually stop the oncoming sheet, drive it backwards, stop it again and subsequently drive it forward. This reverse movement phenomena is a function of blade vibration, revolutions per minute and intersheet drag.
Numerous devices such as impact/paddle feeders of the type disclosed in U.S. Pat. No. 3,630,516 have been employed to minimize the possibility of misfeeds or multifeeds due to the above-mentioned problems.
The aforementioned impact feeders or "inertia feeders" have been employed in top sheet feed devices in an attempt to overcome intersheet friction and assure positive feeding of sheets by jarring or impacting the sheet to be fed from the adjacent sheets. However, in impacting sheets, there is a tendency for the impacting device to jam the sheet to be fed into tighter engagement with the remainder of the sheets in the stack, thereby obviating the benefits obtained in attempting to impact the sheet in the feed direction, off from the remainder of the sheets in the stack. As an improvement, the present top feeder uses an intermittent motion of the paddle wheel, followed by a period of no motion whereby a blade is positioned just at the top sheet surface, followed by a large acceleration with the friction drive force directed as close to parallel to the top sheet surface as possible, followed by an up and away motion from the sheet stack and a relatively constant normal force profile with no impulse spike at the beginning of the arc of contact. This uniform normal force is substantially independent of paddle wheel blade penetration into the top sheet in the stack.